New plugin: loop (#1989)

* New plugin: loop

Add a plugin that detects loops. It does this by sending an unique query
to our selves. If we see the query more than twice we stop the process.
If there isn't a loop, the plugin disables it self and becomes a noop
plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin.cfg

@@ -48,6 +48,7 @@ file:file
 auto:auto
 secondary:secondary
 etcd:etcd
+loop:loop
 forward:forward
 proxy:proxy
 erratic:erratic